For the formation of amorphous silicon films, there have been attempts to use, for example, vacuum vapor deposition process, plasma CVD process, CVD process, reactive sputtering process, ion plating process or light CVD process, and among these processes, the plasma CVD process has been used generally and put to industrial application.
However, for the deposited films constituted with amorphous silicon, there has been room for the further improvement of overall characteristics in electrical and optical characteristics, wear characteristics upon repeated use, use environmental characteristics, as well as productivity and mass productivity including homogenity and reproducibility.
The reaction process in the formation of a deposited amorphous silicon film by using the plasma CVD process that has been put to generalized use is rather complicated as compared with the conventional CVD process, and there have still been several points left unclear in the reaction mechanisms. In addition, there are many parameters for the formation of the deposited film (for instance, substrate temperature, the flow rate and the ratio of gases introduced, inner pressure upon film formation, RF power, electrode structure, structure of reaction vessel, gas discharging rate, plasma generation system, etc.). Combination of these many parameters often renders plasmas instable and causes noticeable undesired effects on the deposited films formed. Moreover, parameters inherent to apparatus have to be selected on every apparatus and, accordingly, generalization for production conditions is difficult at present.
On the other hand, for attaining amorphous silicon films capable of sufficiently satisfying electrical and optical properties as the amorphous silicon films in each of the application uses, it is considered most preferred at present to form them by means of the plasma CVD process.
However, it is necessary to enable mass production with good reproducibility while completely satisfying the increase of area, homogenity of films and homogenity of film quality depending on the application uses of the deposited films. Accordingly, in the formation of a deposited amorphous silicon film by means of the plasma CVD process, an enormous installation cost is required for the mass production apparatus, the items for the administration of mass production thereof are too complicated to restrict the allowable range for the administration and the adjustment of the apparatus is delicate. These have been pointed out as problems to be solved.
As a method of eliminating the foregoing drawbacks in the plasma CVD process, Japanese Patent Laid-Open Sho 60-41047 has proposed a process for the formation of a deposited film on a substrate by separately introducing, into a film-deposition space (A) for forming the deposited film on the substrate, a precursor as the starting material for forming the deposited film which is formed in a decomposition space (B) and an active species which is formed in the decomposition space (C) and which reacts with the precursor respectively.
FIG. 5 schematically shows a typical example of an apparatus which is suitable to practicing the process for the formation of a deposited film.
In FIG. 5, 501 denotes a film-deposition chamber as a film-forming space, in which a deposited film-forming substrate 503 is placed on a substrate bed 502. There are also shown a heater 504 for heating the substrate, starting material gas supply sources 505 through 508 and 512, pressure controllers 505a through 508a and 512a for starting material gas, valves 505b or 505c through 508b or 508c and 512b or 512c for flowing respective gases and mass flow controllers 505d through 508d and 512d for adjusting the flow rate of each of the gases, gas introduction pipes 509 and 513 for respective starting material gases, a microwave source 510 for converting the starting material gas passing through the gas introduction pipe 509 into active species in the decomposition space (C), a waveguide 511 and an electric furnace 514 for converting the starting material gas passing through the gas introduction pipe 513 into a precursor in the decomposition space (B), an exhaustion valve 515, an exhaustion pipe 516 and a quartz pipe 521. The precursor formed in the decomposition space (B) and the active species formed in the decomposition space (C) are reacted with each other in the deposition space (A) for forming a deposited film, thereby forming a deposited film on the substrate 503.
According to the process for the formation of the deposited film, it is possible to easily attain the improvement for the productivity of the film and massproduction thereof while improving various properties, film-forming rate and reproducibility of the films to be formed and unifying the film qualitY, with suitability for increasing the area of the film.
However, when a deposited film is formed on the substrate by the above-mentioned process, the deposited film is deposited not only on the substrate 503 but also on the inner wall of the deposition chamber 501 and on the surface of the substrate support bed 502. Accordingly, it is usual to employ a method of removing the deposited film before transferring to the succeeding film formation by applying dry etching to the inside of the deposition chamber 501 using an appropriate etching gas, for example, a gas mixture of CF.sub.4 and O.sub.2. For instance, dry etching is carried out by a method like that for the formation of active species in the conventional deposited film forming apparatus shown in FIG. 5, introducing an appropriate amount of a gas mixture from a CF.sub.4 /O.sub.2 gas mixture reservoir not illustrated into the deposition chamber 501, charging an appropriate amount of power from a microwave power source 510 and generating based on a gas mixture of CF.sub.4 and O.sub.2 mixed plasmas in the decomposition space (C).
However, the inner wall of the decomposition space (C) or the deposition chamber 501 may sometime be eroded in the dry etching, by F radicals formed, by which elements constituting the inner walls may be made free and adsorbed to the inner wall surface. Further, carbon atoms, oxygen atoms, etc. derived from CF.sub.4 or O.sub.2 may be adsorbed to the inner wall of the decomposition space (C) and the deposition chamber 501.
If the step is transferred to the succeeding filmforming under such conditions, carbon atoms or oxygen atoms adsorbed to the wall surface may be mixed as contaminates into the deposited film during film formation, to form an impurity level and remarkably reduce the properties of the deposited film.
In addition, when activation energy, for example, microwave is introduced into the decomposing space (C) for forming the active species, thereby forming plasmas, the inner wall of the decomposition space (C) is damaged by the plasmas and the inner wall constituent elements (for example, oxygen in the case where the decomposition space (C) is made of quartz (SiO.sub.2 pipe) is made free, which may also be mixed into the deposited film thereby remarkably reducing the property thereof.
Accordingly, since carbon atoms, oxygen atoms, etc. will some time be mixed as contaminants into the film as has been described above, it is further necessary to make an improvement for the process for the formation of the deposited film in order to further satisfy productivity and mass productivity of the film while maintaining various properties of the deposited film.